Closed-loop communication systems have a closed-loop communication link that provides a shared communication channel. A set of communication devices are connected to the closed-loop communication link. Each communication device has un-arbitrated access to the shared communication channel. For example, a facsimile machine, a telephone, and a modem may be connected to a wire that forms a loop throughout a company. The facsimile machine, telephone, and modem each have un-arbitrated access to the wire. Therefore, if the modem is applying modem data to the wire and an individual picks up the telephone, then the voice data will disrupt the modem data and the modem data will disrupt the voice data.
Prior art modems can be broadly classified into two types: (1) 2400 bits per second or less devices, referred to as slow modems, and (2) greater than 2400 bits per second devices, referred to as fast modems. Slow modems may observe an on-demand protocol wherein if there is modem data to transmit, the modem immediately seizes control of the communication link and transmits the modem data to the communication link until there is no longer data to transmit. Fast modems observe a continuous handshake protocol wherein the transmitting modem continuously sends handshake signals to which a receiving modem is to reply. The handshake signals are sent even if a reply is not received. If a reply is received, the two modems communicate with each other.
The slow modem of the prior art presents problems on closed-loop communication links because it automatically seizes control of a closed-loop communication link, even if the receiving modem is not operative. As a result, modem data may unnecessarily disrupt communications on the shared communication channel. Also, slow modems, by definition, can only communicate at 2400 bits per second or less. If faster data rates are desired, slow modems cannot meet the need.
The fast modem of the prior art presents a problem on closed-loop communication links because it continuously sends handshake signals or data. Thus, the fast modem also has the shortcoming of unnecessarily disrupting communications on the shared communication channel.
In view of the foregoing, it would be highly desirable to improve the operation of shared, closed-loop communication links such that the disruption caused by the operation of modems is minimized.